For basic information on the disease and how individuals can mitigate the spread, the following resources provide excellent information:

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  • What is the definition of a "close contact?"


    Both When to Test Calculators are based on CDC guidelines. Current guidelines define a “close contact” as someone who was within six feet of an infected person for a total of at least 15 minutes over the course of 24 hours. One exception to that rule is applicable only to schools: Students who were between three and six feet of an infected person are not considered close contacts as long as they were in school and “correctly and consistently” wearing masks.

    CDC — Close contact definition 

  • There are a lot of different kinds of antigen tests out there. How do I select one for my organization to use?


    In addition to the cost of the individual test kits, the following criteria can inform your choice:

    Accuracy: Most antigen tests have a very high specificity. However, the sensitivity of the different brands of tests varies. The When to Test Calculator for Organizations assumes a lower sensitivity for Antigen tests than with PCR tests, to ensure that the recommended testing frequency is adequate to prevent an outbreak.

    Capital expenditures and maintenance: Some antigen tests run on a small testing instrument, which must be purchased separately and maintained over time. Depending on size of your organization, you may need to purchase several of these instruments in order to be able to test your population quickly enough to prevent an outbreak.

    Staffing requirements: Some antigen tests require trained staff to obtain samples and/or run the tests. If you use an instrument-based antigen test, you will need staff to maintain the instruments, as well. Other types of antigen tests can be performed entirely by the person being tested.

    Time: Some rapid antigen tests can be done at home, decreasing the amount of time taken away from work or school for testing purposes.

    Access: Manufacturing of antigen tests has not always kept up well with demand, so some businesses may find they have a limited selection of brands from which to purchase. Some schools will only have access to a single type of antigen test, which is provided or mandated by the state.

  • What's the difference between PCR and antigen tests?


    PCR tests, also known as molecular tests, look for pieces of the virus’s genetic material using a lab technique called polymerase chain reaction (PCR). They are the most accurate kind of test for COVID-19 currently available.

    PCR samples are usually sent for processing and analysis in a lab, and results can take anywhere from a few hours to several days to arrive. Lab-based PCR tests tend to be more costly than other types of test. Some point-of-care PCR tests are now available, but they typically require purchase of a dedicated instrument and training to run the tests. If you want to use a point-of-care PCR test to screen members of an organization, you may need to purchase multiple instruments and train several staff members for this purpose. Point-of-care PCR tests typically return results in under an hour.

    Antigen tests look for viral proteins. These tests typically provide results within minutes and are very good at accurately identifying people who are shedding a lot of virus particles. They are not as good as PCR tests are at finding people who aren't carrying as much virus, including children and asymptomatic adults. If you are using this type of test in an organizational setting, be sure to understand how well the particular brand you use works in the population you’ll be testing (adults vs. children, people with symptoms vs. people without symptoms). Many antigen tests still require trained staff to run them, but a few are now available over the counter. Antigen tests tend to be the least expensive type of test.

    Because of their decreased sensitivity, antigen tests have the following limitations:

    • If you are using antigen tests to screen members of an organization, you may need to test more frequently than you would if you were using a PCR test in order to prevent an outbreak.
    • Positive results on antigen tests whose specificity is >99% are usually correct, but negative results may need to be confirmed with a PCR test.

    Remember, no test is perfect. 

    Coronavirus Disease 2019 Testing Basics

    CDC - Interim Guidance for Rapid Antigen Testing

  • What is R0 (R-naught)?


    R0 (R-naught) is a measure of the average number of people who will contract a contagious disease from one infected person. It’s a measure of viral transmissibility, or how fast the disease is spreading.

    When R0 is exactly 1.0, it means that, on average, one infected person will infect one other person. When R0 is less than 1.0, it means that the spread of the disease is decreasing within the community. When R0is above 1.0, the spread of disease is increasing. The more transmissible a disease or disease variant is, the higher its R0.

    The When to Test Calculator for Organizations bases its default R0 on CDC estimates regarding the transmissibility of the most prevalent variant of COVID-19 in the US, which is currently the Delta variant. The Calculator's default R0 for Typical conditions reflects the CDC's low-end estimate of Delta's R0, while the default setting for Hotspot conditions reflects a higher-end estimate of Delta's R0. If you know which variant is most common in your area and wish to change the R0 in your scenarios to reflect that variant, click on "Show Advanced Settings," then "Main Calculator Settings,” and scroll down to "Estimated R0."

    CDC — US COVID-19 Cases Caused by Variants

    CDC — What You Need to Know about Variants

    World Health Organization — Tracking SARS-CoV-2 Variants

    Johns Hopkins Medicine — New Variants of Coronavirus


  • What is prevalence?


    For the purposes of the When To Test Calculators, prevalence is the percentage of individuals in a population who are infected with COVID-19 at any given moment. If there are 100 people in a building and one person has COVID-19, then the prevalence of COVID-19 in that building is 1%.

    The prevalence default settings for the When To Test Calculator for Organizations are 1% for typical conditions and 3% for hotspot conditions. To adjust these settings, click on the Show Advanced Settings button on the results page, open the Main Calculator Settings tab, and scroll down to the Prevalence settings (green arrows below).